1. Field of the Invention
This invention relates to a laser beam machine for machining materials, and more particularly, to a laser beam machine using a high-output power CO.sub.2 laser for machining high-reflectivity metals.
2. Description of the Related Art
Current CO.sub.2 lasers have an increased output, and thus are widely used for machining materials or the like.
FIG. 3 is a schematic diagram illustrating the arrangement of a conventional laser beam machine. As shown in the figure, a laser resonator 1 is composed of a total reflection mirror 2 and an output coupling mirror 3, and a laser beam emitted from the laser resonator 1 is focused onto a surface of a workpiece 5 by a light focusing lens 4.
An excitation means for such a laser resonator will be now explained,
FIG. 4(a) and FIG. 4(b) are diagrams illustrating the excitation means for a laser resonator, wherein FIG. 4(a) shows a seed discharge state of the laser resonator, and FIG. 4(b) shows a full-scale discharge state.
Gas lasers utilize an electric discharge for feeding energy into the laser resonator 1; for such a discharge, a high-frequency discharge (RF discharge) is used.
To turn the laser beam on and off at a high speed, the discharge must be switched on and off, and to carry out this operation without the occurrence of an erroneous shot, the discharge is not totally shut-down, but is partly retained in the form of a seed discharge 10, and a full-scale discharge 11 is developed from the seed discharge 10.
The level of such a seed discharge 10 is preferably maintained at as high a level as possible, to ensure a stability of the discharging operation, but if the discharge level becomes too high, a laser output occurs and the surface of the workpiece 5 is damaged. Although this drawback can be eliminated by lowering the level of the seed discharge 10, this can lead to an unstable operation. This can be prevented by maintaining a high level of the seed discharge 10 and reducing the reflectivity of the output coupling mirror 3, but in this case an optimum design of the laser resonator cannot be obtained.
Further, where the material of the workpiece 5 has a high reflectivity (e.g., aluminum, copper, brass, or stainless steel), there is no optimum design capable of suppressing a laser oscillation during the seed discharge, because the total reflection mirror 2, the output coupling mirror 3, the light focusing lens 4, and the surface of the workpiece 5 together form a kind of composite resonator. Namely, if the total reflection mirror 2 and the surface of the workpiece 5 have a reflectivity close to 100%, the value of Q of the composite resonator is increased, and thus the laser oscillation cannot be eliminated regardless of at how low a level the seed discharge is maintained. This is because the focus of the light focusing lens 4 is located on the surface of the high-reflectivity workpiece 5, and therefore, almost 100% of the laser beam output is returned to the laser resonator.
If the above-mentioned phenomenon occurs during a cutting operation, an unwanted scratch is formed on the workpiece 5 upon a machine transfer from a cut portion to a next portion to be cut.
Conventionally, when a laser oscillation occurs in the composite resonator, an external shutter is operated for interception, but this external shutter has large inertia and is unable to follow the on and off cycle of the high-frequency discharge, and thus the machining speed is lowered.